1. Field of the Invention
The present invention relates to a grounding structure in a vehicle provided with a resin body.
2. Description of the Related Art
<Synthetic Resin Vehicle Body>
For a vehicle provided with a metal vehicle body, a grounding structure is common in which the vehicle body and the negative terminal of the battery, and the vehicle body and the GND terminal of the load are connected.
On the other hand, a vehicle provided with a vehicle body integrally molded by using a synthetic resin is disclosed. For such a vehicle, as in JP-A-2009-184611 and JP-A-63-046946, vehicle grounding structures have been proposed in which the negative terminal of the battery and the load GND terminal are connected to a grounding structure element such as a metal plate as a substitute for the metal vehicle body.
<Problem>
<Occurrence of GND Fluctuation>
FIG. 5A and FIG. 5B are views explaining a conventional structure 1 having a grounding structure element; FIG. 5A is a conceptual view of the conventional grounding structure 1 of the grounding structure element and FIG. 5B is an equivalent circuit of the conceptual view of FIG. 5A.
In FIG. 5A, 51 represents a resin vehicle body, 52 represents a grounding structure element provided on the resin vehicle body 51, B represents a battery, L1 represents a first load, and L2 represents a second load. On the resin vehicle body 51 integrally molded by using a synthetic resin, the grounding structure element 52 is present, and on that, the battery B and the two loads (the first load L1, the second load L2) are disposed. The positive terminal + of the battery B is connected to one terminals of the first load L1 and the second load L2, the negative terminal − of the battery B is connected to the grounding structure element 52, and the other terminals of the first load L1 and the second load L2 are connected to the grounding structure element 52.
In this grounding structure, so-called “GND fluctuation” occurs where when a large current flows through either of the first load L1 and the second load L2, the GND potential increases for the other load where no large current flows.
Therefore, the reason that the “GND fluctuation” occurs will be described by using FIG. 5B which is an equivalent circuit of FIG. 5A.
<Reason that GND Fluctuation Occurs>
In FIG. 5B, VB represents a battery voltage, L1 represents the first load, L2 represents the second load, Z12 represents the impedance possessed by the grounding structure element 52 (FIG. 5A) existing between a grounding point of the first load L1 and a grounding point of the second load L2, ZB1 represents the impedance possessed by the grounding structure element 52 existing between a grounding point of the battery B and the grounding point of the first load L1, I1 represents a current flowing from the battery B through the first load L1 and the impedance ZB1, I2 is a current flowing from the battery B through the second load L2, the impedance Z12 and the impedance ZB1. Through the impedance ZB1, the currents I1 and I2 flow.
Therefore, if either of the currents I1 and I2 is a large current, the difference in potential across the impedance ZB1 increases, the potential V1 at a grounding point of the load L1 becomes higher than the potential V0 at a grounding point of the battery voltage VB, the GND potential of the load where a small current flows also becomes the high voltage V1, and here, the “GND fluctuation” occurs.
<Problem of GND Fluctuation>
When the GND fluctuation occurs, since the ground potentials of the first load L1 and the second load L2 become the high voltage V1, for the load where a small current flows, the possibility increases that a malfunction occurs and the possibility increases that electrical characteristics deviate from the rating.
FIG. 6A and FIG. 6B are views explaining a conventional grounding structure 2 having a grounding structure element; FIG. 6A is a conceptual view of the conventional grounding structure 2 of the grounding structure element and FIG. 6B is an equivalent circuit of the conceptual view of FIG. 6A. In FIG. 6A, B represents a battery, Ls represents a signal system load, Ld represents a drive system load, 61 represents a conductive resin vehicle body, 63 represents an insulator, 62M represents a main grounding structure element provided on the insulator 63, 62S represents a signal system grounding structure element, and 62D represents a drive system grounding structure element.
The main grounding structure element 62M is provided through the insulator 63 on the conductive resin vehicle body 61 integrally molded by using a conductive synthetic resin. The battery B, the signal system grounding structure element 62S and the drive system grounding structure element 62D are provided on the main grounding structure element 62M. Further, the signal system load Ls is disposed on the signal system grounding structure element 62S, and the drive system load Ld is disposed on the drive system grounding structure element 62D.
The positive terminal + of the battery B is connected to terminals of the signal system load Ls and the drive system load Ld, the negative terminal − of the battery B is connected to the main grounding structure element 62M at a grounding point B0, the signal system grounding structure element 62S is connected to the main grounding structure element 62M at a grounding point S0, and the drive system grounding structure element 62D is connected to the main grounding structure element 62M at a grounding point D0.
Moreover, the other terminals of the signal system load Ls and the drive system load Ld are connected to the signal system grounding structure element 62S and the drive system grounding structure element 62D at grounding points S1 and D1, respectively.
In the vehicle provided with this grounding structure, the signal system grounding structure element 62S and the drive system grounding structure element 62D which are a plurality of (two in this description) grounding plates are placed on the main grounding structure element 62M connected to the negative terminal − of the battery B in the vehicle. Then, the signal system grounding structure element 62S and the drive system grounding structure element 62D, and the signal system load Ls and the drive system load Ld are connected to the positive terminal + of the battery B through electric wires. Moreover, the signal system grounding structure element 62S and the drive system grounding structure element 62D are attached to the vehicle while being insulated from the conductive resin vehicle body 61.
By thus separately disposing the signal system grounding structure element 62S and the drive system grounding structure element 62D as grounding plates, no large current from the drive system load Ld of the drive system grounding structure element 62D flows to the side of the signal system grounding structure element 62S, so that the occurrence of the GND fluctuation arising from wiring resistance can be restrained. For this reason, the potential of the grounding circuit of the signal system is stabilized, so that a grounding structure resistant to the GND fluctuation can be realized.
<Reason that GND Fluctuation Occurs>
However, a verification has found that there still is a possibility of occurrence of the GND fluctuation even with this. The reason therefor will be described with reference to FIG. 6B which is an equivalent circuit of FIG. 6A.
In FIG. 6B, VB represents a battery voltage, Ls represents the signal system load, Ld represents the drive system load, Z2s represents the impedance possessed by the signal system grounding structure element 62S existing between the connection point S1 (FIG. 6A) of the signal system grounding structure element 62S on the side of the signal system load Ls and the connection point S0 (FIG. 6A) of the signal system grounding structure element 62S on the side of the main grounding structure element 62M, Z2d represents the impedance possessed by the drive system grounding structure element 62D existing between the connection point D1 (FIG. 6A) of the drive system grounding structure element 62D on the side of the drive system load Ld and the connection point D0 (FIG. 6A) of the drive system grounding structure element 62D on the side of the main grounding structure element 62M, Zsd represents the impedance possessed by the main grounding structure element 62M existing between the connection point S0 (FIG. 6A) of the main grounding structure element 62M (FIG. 6A) on the side of the signal system grounding structure element 62S and the connection point D0 (FIG. 6A) on the side of the drive system grounding structure element 62D, ZBs represents the impedance possessed by the main grounding structure element 62M existing between the connection point B0 (FIG. 6A) of the main grounding structure element 62M (FIG. 6A) on the side of the battery and the connection point S0 (FIG. 6A) on the side of the signal system grounding structure element 62S, I1s represents a current flowing from the battery voltage VB through the signal system load Ls, the impedance Z2s and the impedance ZBs, and I1d represents a current flowing from the battery voltage VB through the drive system load Ld, the impedance Z2d, the impedance Zsd and the impedance ZBs. The currents I1s and I1d flow through the impedance ZBs.
Therefore, if the current I1d flowing through the drive system load Ld is a large current or a current with large fluctuation, the difference in potential across the impedance ZBs increases and the voltage V11 of the impedance ZBs on the side of the impedance Zsd becomes higher than the potential V0 at the grounding point of the battery voltage VB, so that the “GND fluctuation” where the GND potential of the signal system load Ls becomes a high voltage occurs.
If the GND fluctuation occurs, particularly, the possibility increases that a malfunction occurs on the signal system load Ls, and the possibility increases that electrical characteristics deviate from the rating.